Stencil printing is a printing process which can be easily carried out and has been extensively used from old times. As a result of the automation of stencil preparation and printing operation in stencil printing in recent years, this printing process has become more convenient and has come to be widely used in new applications. For this stencil printing, W/O emulsion inks have been mostly used from the standpoints of printability and machine operation. Such inks are usually produced by kneading a colorant (coloring material) together with a resin and adding water thereto.
However, formulations should be varied according to colorants in order to impart satisfactory storage stability to the inks, and much labor has been required for making particular formulations suitable for obtaining satisfactory inks. Since the storage stability of emulsion inks varies considerably depending on the chemical properties of individual colorants, colorant suitability should be carefully tested when selecting a colorant. In addition, the kinds of usable colorants have been limited accordingly.
Furthermore, there are cases where some kinds of colorants are unable to give inks having satisfactory storage stability even when formulations are changed. Another drawback of conventional emulsion inks is that according to the degree of kneading of the emulsion ink during continuous printing, the amount of colorant components which dissolve into the aqueous phase or oil phase changes or the interaction between the colorant and components of the oil phase of the emulsion ink changes, and this results in fluctuations in the viscosity of the emulsion ink.